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PubMed1993

Thermoregulatory response thresholds during spinal anesthesia.

Reportedly, during spinal anesthesia, the shivering threshold is reduced approximately 1 degree C but the vasoconstriction threshold remains normal. Such divergence between the shivering and vasoconstriction thresholds is an unusual pattern of thermoregulatory impairment and suggests that the mechanisms of impairment during regional anesthesia may be especially complex. Accordingly, we sought to define the pattern of thermoregulatory impairment during spinal anesthesia by measuring response thresholds. Seven healthy women volunteered to participate on two study days. On one day, we evaluated thermoregulatory responses to hypothermia and hyperthermia during spinal anesthesia; on the other day, responses were evaluated without anesthesia. Upper body skin temperature was kept constant throughout the study. The volunteers were warmed via the lower body and cooled by central venous infusion of cold fluid. The core temperatures triggering a sweating rate of 40 g.m-2 x h-1, a finger flow of 0.1 mL/min, and a marked and sustained increase in oxygen consumption were considered the thermoregulatory thresholds for sweating, vasoconstriction, and shivering, respectively. Spinal anesthesia significantly decreased the thresholds for vasoconstriction and shivering, and the decrease in each was approximately 0.5 degree C. The range of temperatures not triggering thermoregulatory responses (those between sweating and vasoconstriction) was 0.9 +/- 0.6 degree C during spinal anesthesia. The synchronous decrease in the shivering and vasoconstriction thresholds during spinal anesthesia is consistent with thermoregulatory impairment resulting from altered afferent thermal input.

Adult↗
PubMed2005

Energetics of a long-distance migrant shorebird (Philomachus pugnax) during cold exposure and running.

The metabolic consequences of cold exposure and exercise are not well characterized in birds. Ruff sandpipers Philomachus pugnax are migrant shorebirds traveling between Africa and Siberia for up to 30,000 km annually. Our goal was to quantify the fuel selection pattern of these remarkable athletes during shivering and terrestrial locomotion. We used indirect calorimetry and nitrogen excretion analysis to measure their rates of lipid, carbohydrate and protein oxidation at different temperatures (22, 15, 10 or 5 degrees C) and different treadmill speeds (15, 20, 25, 30, 35 or 40 m min(-1)). Results show that lipid oxidation supplies nearly all the energy necessary to support shivering and running, and that the pattern of oxidative fuel selection is independent of shivering or running intensity. During shivering, total ATP production is unequally shared between lipids (82%), carbohydrates (12%) and proteins (6%). During running, lipids remain the dominant substrate (66%), with carbohydrates (29%) and proteins (5%) playing more minor roles. The prevailing use of lipids during intense shivering and high-speed running is not consistent with the fuel selection pattern observed in exercising and cold-exposed mammals. The exact mechanisms allowing birds to use lipids at extremely high rates are still largely unexplored, and quantifying the relative importance of different fuels during long-distance flight remains a major challenge for future research.

Acclimatization↗
PubMed1995

Influence of body composition on rewarming from immersion hypothermia.

BACKGROUND: This study was conducted to determine if the differences between efficacies of three treatments for immersion hypothermia are affected by body composition. METHODS: Twelve subjects were divided into equally sized low (LF) and high (HF) fat groups. On three occasions subjects were each immersed in cold water until esophageal temperatures (Tes) decreased to approximately 33.2 degrees C (LF) and approximately 35.8 degrees C (HF). They were then rewarmed by: 1) shivering; 2) application of external heat; or 3) treadmill exercise in a balanced design. RESULTS: For HF, the afterdrop during exercise (1.04 +/- 0.2 degrees C) was greater than during shivering (0.35 +/- 0.3 degrees C) and external heat (0.36 +/- 0.1 degree C) (p < 0.01). In LF, however, the exercise afterdrop (0.75 +/- 0.2 degree C) was greater than only external heat (0.35 +/- 0.2 degree C) (p < 0.05) but not shivering (0.58 +/- 0.4 degree C). There was a positive relationship between % fat and afterdrop for the exercise condition with a slope (95% C.I.) of 0.03 (0.01 to 0.05) degree C.% fat-1 (r2 = 0.37, p < 0.05). The exercise rewarming rate (3.48 +/- 1.1 degrees C.h-1) was greater (p < 0.01) than during both shivering (1.80 +/- 0.7 degrees C.h-1) and external heat (2.22 +/- 0.7 degrees C.h-1) in HF while no difference was seen between the three treatments (5.28 +/- 0.4, 4.86 +/- 1.1 and 5.16 +/- 0.7 degrees C.h-1, respectively) in LF. There were inverse relationships between % fat and rewarming rate in the exercise -0.12 (-0.23 to -0.01) degree C.h-1.% fat-1, (r2 = 0.38), shivering -0.27 (-0.38 to -0.16) degrees C.h-1.% fat-1, (r2 = 0.76) and external heat -0.26 (-0.35 to -0.17) degree C.h-1.% fat-1, (r2 = 0.83) conditions (p < 0.05). CONCLUSIONS: The inter-treatment differences between these techniques are accentuated in the HF, and attenuated (afterdrop) or even eliminated (rewarming rate) in the LF subgroup.

Adult↗
PubMed1995

Gender differences in physiological reactions to thermal stress.

Following an extensive anthropometric evaluation, thermoregulatory responses were studied in nine men and nine women who performed immersed exercise with post-exercise rest in 28 degrees C water. During the post-exercise period esophageal temperature (Tes), oxygen consumption, heat flux and skin blood perfusion were monitored at 10 s intervals, with average minute values used for calculations. The delta Tes (relative to resting Tes) at which sweating abated and shivering commenced were defined as the delta Tes thresholds for the cessation of sweating and onset of shivering, respectively. No significant gender differences were evident in the sweating and shivering threshold delta Tes values, or the magnitude of the null-zone. Using z-tests for parallelism the rates of core cooling across the null-zone were not found to differ significantly between genders, nor were the slopes of the perfusion: delta Tes responses across the null-zone or the post-threshold shivering responses (ml.kg-1.min-1.degrees C-1). The slope of the sweating response (measured from immersion until sweat cessation; g.m-2.min-1 degree C-1) was, however, significantly lower in the female than in the male samples (z = 3.93; P < 0.01). Despite the gender-related dimorphic distribution of adipose tissue, both men and women lost equal proportions of their total heat flux from central and peripheral measurement sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1996

Passive temperature lability in the elderly.

Thermoregulatory responses of nine healthy elderly [seven men and two women; mean age (SD) 73.9 (4.8) years] were compared to those of nine young adult men [26.6 (5.2) years]. They exercised on a cycle ergometer for 20 min at an intensity inducing a heart rate equivalent to 65% of their predicted maximum, and were thereafter immersed in 28 degrees C water. The exercise was conducted to elevate tympanic temperature (Tty) and initiate a steady rate of sweating. The post-exercise immersion period induced gradual cooling of Tty, and changes in Tty relative to resting levels (delta Tty) at which sweating abated and shivering commenced were defined as the delta Tty thresholds for the cessation of sweating (Tsw) and onset of shivering (Tsh), respectively. In addition to Tty, oxygen uptake (VO2; l.min-1), sweating rate (g.m-2.min-1), and forehead skin blood perfusion were also measured during the trials. The mean (SD) Tsw occurred at a significantly (P < 0.005) higher delta Tty [0.48 (0.18) degrees C] in the elderly than in the young adults [0.21 (0.06) degrees C], while the Tsh occurred at significantly (P < 0.005) lower delta Tty in the elderly [-0.64 (0.34) degrees C] than in young adults [-0.22 (0.10) degrees C]. Decreases in delta Tty below the shivering threshold were met with a significantly (P < 0.01) reduced VO2. The range of temperature lability between Tsw and Tsh, defined as the null-zone, was significantly greater in the elderly [1.12 (0.39) degrees C] than in the young adults [0.43 (0.12) degrees C], and the slope of the vasoconstrictor response in the null-zone was significantly (P < 0.001) lower in the elderly subjects. The present study demonstrates a greater passive core temperature lability in older individuals, since the effector responses of sweating and shivering were initiated at higher and lower levels of Tty, respectively. The magnitudes of the effector responses beyond the thresholds were also significantly reduced, suggesting that the elderly may be more susceptible to hypo-/hyperthermia during periods of endogenous and/or exogenous thermal stress.

Adult↗
PubMed1989

Postoperative ventilatory and circulatory effects of extended rewarming during cardiopulmonary bypass.

Postoperative effects of extended rewarming (ECR) after hypothermic cardiopulmonary bypass (CPB) were studied. All (n = 28) patients were rewarmed to a nasopharyngeal temperature exceeding 38 degrees C before terminating CPB. In 12 patients (control group) the rectal temperature (Tre) was 33.8 +/- 1.7 degrees C (mean +/- sd) at termination of CPB. In sixteen patients (ECR group) rewarming during CPB was continued to a Tre of 36.8 +/- 0.5 degrees C. Postoperative body temperatures, heat content, shivering, oxygen uptake, CO2 production and haemodynamic variables were measured. ECR reduced the heat gain required to complete core rewarming to 665 +/- 260 kJ, compared with 1037 +/- 374 kJ in the control group (p less than 0.01). The incidence of shivering was reduced (p less than 0.05) as well as shivering intensity and duration. In seven non-shivering ECR group patients this coincided with significantly reduced metabolic and ventilatory demands but these improvements were not valid for the group as a whole. The required ventilation temporarily during postoperative rewarming in both groups increased to 250 per cent of the basal need. Extending CPB rewarming (to at least 36 degrees C Tre) was inefficient when used as the sole measure to reduce the untoward effects of residual hypothermia during recovery after cardiac surgery with hypothermic CPB.

Adult↗
PubMed1996

[The effect of convection warming during abdominal surgery on the early postoperative heat balance].

UNLABELLED: Hypothermia (core temperature < 36 degrees C) is common after longer-lasting surgical procedures. Heat loss mainly occurs during anaesthesia and surgery and leads to increased risk, especially in the early recovery period of elderly patients. In the present study we investigated the effects of intraoperative forced-air warming, administered via an upper-body blanket ("Warm Touch", Mallinckrodt, USA), with the specific aims of: (1) drawing up heat balances; and (2) analysing postoperative thermoregulation, oxygen consumption (VO2) and cardiovascular reactions of mechanically ventilated patients. The general aim of our study was to compare intraoperative forced-air-warming and conventional patient-insulation with cotton blankets. METHODS: Twenty four ASA II and III patients scheduled for elective colon surgery were randomly assigned to a control group (n = 12, no warming therapy, upper body covered with a cotton hospital blanket) or a convective warming group (n = 12). Anaesthesia was administered with etomidate (0.2 mg/kg), fentanyl (approximately 10 micrograms/kg) and vecuronium bromide (0.1 mg/kg). During surgery the lungs were mechanically ventilated with 70% nitrous oxide in oxygen and enflurane (end-tidal-concentration max. 0.7%) using a semiclosed circuit with a fresh gas flow of 3 l/min. A hygrophobe heat and moisture exchanger ("Sterivent," Darex Corp., Italy) was used. At the end of surgery patients were transferred to the ICU, covered with a hospital cotton-quilt and normo-ventilated using a Bennett 7200 a. Patients were sedated/kept free of pain by administering titrated doses of midazolam and/or piritramide. Postoperative oxygen consumption (VO2) was recorded continuously with a Deltatrac Metabolic Monitor (Datex Corp., Finland). Pre-, intra- and postoperative measurements included heart rate, invasive blood pressure, core-temperature (before and after operation: urinary bladder-temperature, during surgery: oesophageal temperature) and mean-skin-temperature (according to Ramanathan) up to 180 min from the end of surgery. Shivering, pharmacological interventions (e.g. pethidine) and time of extubation were noted. Data are presented as median, minima and maxima. The results were analysed using the Mann-Whitney U test or Chi-Square test (shivering). Statistical significance was assumed when P < 0.05. RESULTS: Both groups were comparable for gender, body weight, height, age, duration of their operations and amount of intraoperative fluids, narcotics and muscle relaxants. Room temperatures in the control group were significantly higher than in the forced air group (24 vs 22 degrees C). Initial setting of the forced-air blower was "high" (42-46 degrees high air flow). When the oesophageal-temperature reached 36.5 degrees C, the blower temperature was reduced to 36-40 degrees C. Reduction was necessary approximately 60 min from start in the operation. At the end of surgery/administration to the ICU core-temperatures of both groups differed significantly (35.2/ 35.4 degrees C vs 36.3/36.2 degrees C). Mean-skin temperatures were higher, too, but no statistical analysis was carried out for the intraoperative period, because warm air influenced skin thermometers located on the upper body. At admission to the ICU patients in the control group had a heat loss of 4.4 kJ/kg; those in the convective warming group had a heat-gain of 0.8 kJ/kg. Further measurements of postoperative core temperatures did not differ significantly, but the skin-temperatures of patients who received forced-air warming in the theatre remained higher (P < 0.05) until 120 min from the end of surgery. Shivering was more frequent and lasted longer in the control group (8 patients, 20 min vs 4 patients, 9 min; P < 0.05). Patients in the control group needed more drugs to stop increased cardiovascular reactions (hypertension, tachycardia) or shivering.(ABSTRACT TRUNCATED)

Adult↗
PubMed1996

Thermosensitivity is reduced during fever induced by Staphylococcus aureus cells walls in rabbits.

Thermosensitivity (TS) and threshold core temperature for metabolic cold defence were determined in six conscious rabbits before, and at seven different times after i.v. injection of killed Staphylococcus aureus (8 x 10(7) or 2 x 10(7) cell walls x kg(-1)) by exposure to short periods (5-10 min) of body cooling. Heat was extracted with a chronically implanted intravascular heat exchanger. TS was calculated by regression of metabolic heat production (M) and core temperature, as indicated by hypothalamic temperature. Threshold for cold defence (shivering threshold) was calculated as the core temperature at which the thermosensitivity line crossed preinjection resting M. The shivering thresholds followed the shape of the fever response. TS was significantly reduced (up to 49%) during the time course of fever induced by the highest dose of pyrogen only. At both high and low doses of pyrogen TS correlated negatively with shivering threshold (r = 0.66 and 0.79 respectively) with similar slopes. The reduction in TS during fever was thus associated with the increase in shivering threshold resulting from the pyrogen injection and not by the dose of pyrogen. Model considerations indicate, however, that changes in sensitivity of the thermosensory input to the hypothalamic controller may affect threshold changes but cause negligible TS changes. It is more likely that the reduction in TS is effected in the specific hypothalamic effector pathways.

Animals↗
PubMed1995

Noradrenaline-induced hypothermia is suppressed in the vagotomized cold-exposed pigeon.

Vagotomized (VX) pigeons studied 2 days after surgery exhibited a significant decrease in cloacal temperature (Tb) and respiratory rate (Rf), and an increase in heart rate (Hf) and metabolic rate (M) at the thermoneutral zone, when compared with sham-operated (SVX) pigeons. The effect of intravenous noradrenaline (NA) on Tb, Rf, Hf and M was examined in SVX and VX-pigeons at 15 degrees C. Following NA administration, the Tb and Rf in the SVX-birds dropped from the preinjection level, but in VX-pigeons, they were not significantly altered. In SVX-pigeons, a total suppression of shivering was apparent following NA-injection, while in the majority of VX-pigeons, shivering was only slightly affected or even increased. The Hf in both SVX- and VX-birds increased following NA-injection. The responses to reserpine were qualitatively similar to NA, although much slower. There were no differences between SVX- and VX-birds with regard to Tb, M and Rf following acetylcholine (ACh) and eserine (Ese) injection. Hf increased after ACh+Ese administration in SVX-pigeons, but in VX-birds, it decreased after an initial surge. Shivering was suppressed for 18-20 min in SVX-birds and 30-50 min in VX-birds. It is suggested that the lack of the hypothermic effect of NA in VX-birds is due to the maintenance of oxygen uptake with unimpaired capacity for shivering.

Acetylcholine↗
PubMed2005

Relationships among the behavioral, noradrenergic, and pituitary-adrenal responses to interleukin-1 and the effects of indomethacin.

Peripheral administration of interleukin-1 (IL-1) is known to activate the hypothalamo-pituitary-adrenal axis (HPA axis) and brain noradrenergic systems. We studied the relationship between these responses using in vivo microdialysis to assess the release of hypothalamic norepinephrine (NE), while simultaneously sampling blood for ACTH and corticosterone, and monitoring body temperature and behavior in freely moving rats. Rats were implanted with microdialysis probes in the medial hypothalamus, with intravenous catheters, and with telethermometers in the abdomen. Each rat was injected with saline and IL-1beta (1 microg ip) in random order, monitoring microdialysate NE, body temperature and plasma ACTH and corticosterone for 2-4 h after injection. Saline injections were followed by transient increases in microdialysate NE and in plasma ACTH and corticosterone. IL-1beta injections resulted in prolonged elevations of microdialysate NE, as well as plasma ACTH and corticosterone, and body temperature. IL-1beta also induced shivering and a prolonged depression of locomotor activity. Pretreatment with indomethacin (10 mg/kg sc) prevented the IL-1beta-induced increases in body temperature and the apparent increase in hypothalamic NE release, but only attenuated the IL-1beta-induced shivering and the increase in plasma ACTH. The results indicate a close temporal relationship between the release of NE and HPA axis activation. Such a relationship is also supported by the similar effects of indomethacin pretreatment on NE and ACTH. The shivering is likely involved in the increase in body temperature, but indomethacin only attenuated the shivering while it blocked the fever. However, the effects of indomethacin clearly indicate that neither the increase in body temperature nor the increase in hypothalamic NE release was essential for HPA axis activation. These results suggest that hypothalamic NE is involved in the IL-1-induced HPA axis activation, but that this is not the only mechanism by which the HPA axis is activated by intraperitoneally injected IL-1.

Adrenocorticotropic Hormone↗
PubMed1981

Evidence for a caudal brainstem site of action for cannabinoid induced hypothermia.

delta 9-Tetrahydrocannabinol (THC), 11-hydroxy delta 9-tetrahydrocannabinol (11-OH-THC) and the synthetic dimethylheptyl analogue of THC (DMHP) were injected intracerebrally into proven chemosensitive sites in the hypothalamus of unanesthetized cats with implanted microinjection guide tubes. 100 micrograms of each compound was administered in a volume of 8 microliters. Chemosensitivity of all injection sites was established by microinjection of carbamylcholine to induce hyperthermia and tetrodotoxin to induce hypothermia. THC or its analogues produced no significant change in body temperature when injected intracerebrally. However, in the same animals, parenteral administration of THC, 11-OH-THC or DMHP (0.5 to 2.0 mg/kg) induced hypothermic responses ranging from -2.0 to -7.0 degrees C. Intravenous administration of THC was effective in blocking shivering induced by cooling the preoptic region in unanesthetized cats with implanted thermodes. In cats with mid-pontine transections, cooling of the spinal cord by perfusion with an epidural double wall cannula at temperatures of 30, 20, 10 and 0 degrees C produced graded shivering which was recorded electromyographically. Intravenous THC, (0.25-2.0 mg/kg) produced a dose-dependent attenuation of spinal cord induced shivering. These data plus results of prior studies suggest that the tetrahydrocannabinols produce their hypothermic effect at sites in the caudal brainstem. Suppression of shivering at the ponto medullary or spinal cord level may represent an important mechanism which contributes to the lowering of body temperature.

Animals↗
PubMed2000

Effects of intracerebroventricular injections of des-His1 (Glu9) glucagon amide on the regulatory thermogenesis in muscovy ducklings.

Recent investigations have demonstrated a modulatory action of glucagon on shivering via the central nervous system in ducklings. Such an action could be mediated by glucagon receptors that have been recently detected in several brain areas involved in the central control of the involuntary motricity in this avian species. The present study using des-His1 (Glu9) glucagon amide, was performed to investigate the central mechanisms of glucagon on shivering. This glucagon analog was found to be an antagonist of glucagon devoid of adenylate cyclase activity (GR2) by triggering the breakdown of inositol phosphate (GR1) in mammals hepatocytes. The intracerebroventricular administration of des-His1 (Glu9) glucagon amide or glucagon induced a marked inhibition of shivering in ducklings exposed to cold. It seems likely that GR1 receptors contribute to decreased shivering in ducklings exposed to cold. Central glucagon or des-His1 (Glu9) glucagon amide were devoid of thermogenic effect at thermoneutrality.

Animals↗
PubMed2003

Preoperative combined with intraoperative skin-surface warming avoids hypothermia caused by general anesthesia and surgery.

STUDY OBJECTIVES: To evaluate the effects of intraoperative skin-surface warming with and without 1 hour of preoperative warming, in preventing intraoperative hypothermia, and postoperative hypothermia, and shivering, and in offering good conditions to early tracheal extubation. DESIGN: Prospective, randomized, blind study. SETTING: Teaching hospital. PATIENTS: 30 ASA physical status I and II female patients scheduled for elective abdominal surgery. INTERVENTIONS: Patients received standard general anesthesia. In 10 patients, no special precautions were taken to avoid hypothermia. Ten patients were submitted to preoperative and intraoperative active warming. Ten patients were only warmed intraoperatively. MEASUREMENTS AND MAIN RESULTS: Temperatures were recorded at 15-minute intervals. The patients who were warmed preoperatively and intraoperatively had core temperatures significantly more elevated than the other patients during the first two hours of anesthesia. All patients warmed intraoperatively were normothermic only at the end of the surgery. The majority of the patients warmed preoperatively and intraoperatively or intraoperatively only were extubated early, and none had shivering. In contrast, five unwarmed patients shivered. CONCLUSIONS: One hour of preoperative warning combined with intraoperative skin-surface warming, not simply intraoperative warming alone, avoided hypothermia caused by general anesthesia during the first two hours of surgery. Both methods prevented postoperative hypothermia and shivering and offered good conditions for early tracheal extubation.

Abdomen↗
PubMed2004

Mood change after anaesthesia with remifentanil or alfentanil.

BACKGROUND AND OBJECTIVE: There are anecdotal reports of dysphoria occurring in patients on the first day after anaesthesia with remifentanil. This study was performed to investigate this allegation and to find a possible relationship to postoperative shivering or to nausea and vomiting. METHODS: Patients undergoing otorhinolaryngeal surgery took part in a prospective, randomized, double-blind study comparing total intravenous anaesthesia with propofol (2 mg kg(-1) bolus injection then 100 microg kg(-1) min(-1)) and remifentanil (1 microg kg(-1) bolus then 0.1-0.5 microg kg(-1) min-1) or alfentanil (30 microg kg(-1) bolus then 0.16-0.83 microg kg((-1) min(-1)). The patients were carefully insulated and actively warmed by convective heating and rectal temperature was monitored continuously. Postoperative shivering was graded on a three-point scale, and the cumulative incidence of nausea and vomiting were registered at 24 h after surgery. Pre- and postoperative mood was measured with the von Zerssen mood scale (Befindlichkeits-Skala) and changes tested for significance. High scores reflect discontent and dysphoria. RESULTS: The data of 98 patients (49 in each group, ASA I-II, age 42 +/- 13 yr, anaesthesia time 141 +/- 60 min; mean +/- SD; intergroup P values > 0.1) were evaluated. Core temperature did not change perioperatively (before 36.6 +/- 0.2 degrees C; after 36.8 +/- 0.3 degrees C, inter- and intragroup P > 0.1). The incidence of nausea was the same in each group; vomiting occurred with equal frequency (6/49 vs. 7/49). Shivering was significantly more frequent after remifentanil (41% vs. 10%, P < 0.001). The patients' mood remained stable after remifentanil but worsened after alfentanil (von Zerssen score from 9.3 +/- 2.5 to 13.9 +/- 3.6; mean +/- 95% confidence intervals; P < 0.01). DISCUSSION: Postoperative shivering was more frequent after remifentanil but was unrelated to intraoperative heat loss. Contrary to preliminary informal observations, there was no evidence that remifentanil caused postanaesthetic dysphoria on the day one after surgery.

Adult↗
PubMed1979

Accidental hypothermia and rewarming in dogs.

1. Twenty lightly anaesthetized dogs were cooled to 29 degrees C by cold-water immersion. Ventilation was spontaneous and the animals were allowed to shiver freely. Metabolic heat production and respiratory heat exchange were measured during rewarming. 2. The animals were divided into four groups each of five dogs and each group was rewarmed by a different technique. The control group was allowed to rewarm spontaneously; a second group was given warm (45-50 degrees C) fully humidified air to breathe in addition; a third group was rewarmed in a hot-water bath (42-44 degrees C) and the remaining group was given in muscle relaxant to abolish shivering and rewarmed by warm inspired air only. 3. The group rewarmed in hot water achieved normal core temperature most rapidly but there was no difference in the rewarming rates of the group rewarmed spontaneously and of the group given warm air to breathe in addition. 4. The group given a muscle relaxant and rewarmed with warm inspired air required 12 h to achieve the same core temperature as the shivering groups achieved in 2 h. Compared with the heat produced by shivering the amount of heat which it was possible to transfer across the respiratory tract was so small that it did not materially influence the rate of rewarming.

Animals↗
PubMed1982

Influence of respiratory heat transfer on thermogenesis and heat storage after cold immersion.

1. Ten male subjects were cooled on three occasions to a rectal temperature of 35 degrees C by immersion to the neck in water at 11.3 degrees C. The subjects were rewarmed for 60 min, once by metabolic heat production alone (shivering), once by inhalation rewarming with spontaneous breathing of saturated air at 47 degrees C (control) and once by inhalation rewarming with ventilation regulated at 40 litres/min by respiring a controlled fraction of CO2 (hyperventilation). 2. Metabolic heat production was substantially reduced by inhalation rewarming (P less than 0.05), from 913 kJ when shivering to 766 kJ (control) and 613 kJ when hyperventilating. The fall in metabolic heat production was greater than the corresponding respiratory heat gain, which increased from a loss of 41 kJ when shivering to gains of 85 kJ (control) and 169 kJ (hyperventilation). 3. As differences in mean skin temperatures were small (less than 1.0 degrees C), it is concluded that the lower metabolic heat production in response to increased respiratory heat input must result from more rapid central temperature gains. This conclusion is supported by the relative values of rectal and tympanic temperatures. It was calculated that the percentage of the total heat supply which was donated to the core increased from 13% during shivering to 16% for the control and 23% in hyperventilation. Results imply that respiratory heat input is more efficient than metabolic heat production in elevating central temperature.

Adult↗
PubMed1996

Thermoregulation and heart rate variability.

1. Heart rate variability is modulated by multiple control systems, including autonomic and hormonal systems. Long-term variability, i.e. the very low-frequency band of the power spectra, has been postulated to reflect thermoregulatory vasomotor control, based upon thermal entrainment experiments. However, the relationship between thermoregulatory responses (vasoconstriction and shivering) and heart rate variability has not been studied. 2. We performed two distinct protocols in a series of human subjects. In the first protocol, core temperature was reduced by intravenous infusion of cold saline, while skin temperature was unchanged. The second protocol involved skin-surface warming and cooling until shivering developed. Power spectral analysis was performed using a fast Fourier transformation, and the area in three distinct band-widths was determined. 3. Very low-frequency power (0.0039-0.04 Hz) increased significantly in response to core cooling, peripheral vasoconstriction and shivering, while both very low- and low- (0.04-0.15 Hz) frequency power increased in response to skin-surface cooling. Heart rate decreased during core cooling-induced vasoconstriction, suggesting a direct thermal response, and increased in relation to the metabolic demands associated with shivering. 4. Our results suggest that very low-frequency power is modulated by thermal stimuli which result in core hypothermia and thermoregulatory activity, while skin-surface cooling without core hypothermia does not selectively modulate this frequency band.

Adult↗
PubMed1996

Relation between systemic oxygen uptake and tissue oxygen extraction following cardiac surgery.

Twenty-three patients undergoing elective coronary artery bypass graft surgery with moderate hypothermic bypass, under opiate-benzodiazepine anaesthesia were investigated during the first 7 h following surgery. Patients with recent myocardial infarct (< 6 weeks), reduced left ventricular ejection fraction (< 40%) or requiring inotropic support were excluded. During the first 3 h after surgery, higher oxygen uptake and oxygen extraction ratio were found in shivering patients (n = 5) compared with non-shivering patients (N = 16). A significant relation was found between oxygen uptake and the oxygen extraction ratio for the pooled data (R = 0.80 and 0.87, in shivering and non-shivering patients, respectively) as well as for the individual patients. A weaker relation was observed between cardiac output and oxygen uptake. The lack of adequate circulatory compensation could be related to the myocardium, surgical trauma, cardioplegic arrest and to peripheral vasoconstriction.

Anesthesia, General↗